A fiber-reinforced plastic (FRP) material, which is a composite material of fiber, for example, glass fiber or carbon fiber and a plastic, is a material that has long been used for sports and leisure, such as tennis rackets, bicycles, and fishing rods, by taking advantage of its features of being lightweight and having high strength and high rigidity. In recent years, applications of the fiber-reinforced plastic material have been continuously expanding, and its development ranges from consumer equipment to industrial equipment, for example, from housings of electronic devices, such as notebook PCs and tablets, to arms of industrial robots and the like, and reinforcing materials for architectural structures.
Further, due to a recent rise in crude oil price and increase in awareness of global environmental protection, there is a strong demand for energy savings and resource savings. Particularly in transportation equipment, such as automobiles and aviation equipment, in which fossil fuel is used, lowering of fuel consumption has been actively promoted. The lowering of the fuel consumption of the transportation equipment is extremely greatly affected by lightweighting of a vehicle body, and hence an FRP using carbon fiber has started to be used in such applications in place of a metal material.
The FRP material is produced by impregnating a reinforcing fiber base material with a liquid matrix resin, followed by curing. As the liquid resin that the reinforcing fiber base material is to be impregnated with, there has been mainly used a thermosetting resin, for example, an epoxy resin because of ease with which the fiber base material is impregnated with the resin. However, when the thermosetting resin is used as the matrix resin, it is generally essential to use a curing agent in combination therewith, and hence there are problems in that: such mixture has a large storage load and requires a long curing time, and hence productivity is low; and the mixture lacks recyclability like that of the metal material. There is a strong demand for amelioration of the problems. As an FRP molding material, there has been generally used a prepreg, which is obtained by dissolving the thermosetting resin in a solvent together with the curing agent, impregnating the reinforcing fiber base material with the solution, and then retaining the resultant in a heated and semicured (B-stage) state. However, the prepreg has had the above-mentioned problems.
Accordingly, in Patent Literature 1, there is a proposal that an FRP molding prepreg be obtained by melt-kneading a solid epoxy resin having a softening point (Ts) of 50° C. or more and a melt viscosity at 150° C. measured with a cone-plate viscometer of 500 mPa·s or less, another bisphenol-type solid epoxy resin, a tetracarboxylic acid dianhydride, and a curing accelerator to provide an epoxy resin composition, pulverizing the epoxy resin composition into powder, and applying the powder to a reinforcing fiber base material, followed by heating and melting. However, in this technique, it is essential that two kinds of different solid epoxy resins be used in combination as the epoxy resin serving as the matrix resin, and besides, the curing agent is used. Accordingly, as shown in Examples, even with the use of the curing accelerator, a curing time is as long as 1 hour. Besides, a cured product of the matrix resin has a glass transition point (Tg) of 150° C. or less, and hence its heat resistance is insufficient. In addition, in a curing reaction using an epoxy resin monomer that is a low-molecular-weight species, the following problems may occur: burr and dripping resulting from low viscosity; and molding failures, such as curing shrinkage and strain due to a curing reaction because of many reaction sites (functional groups). Thus, molding is not as easy as with a thermoplastic resin, and hence it is considered that the technique is not suitable for molding processing of a housing having a complicated shape like an electronic device.
Meanwhile, there has also been considered a technique involving using, for the matrix resin, a thermoplastic resin, which does not require a curing reaction, in place of the thermosetting resin, to thereby solve the problems. For example, in Patent Literature 2, there is proposed a prepreg of a fiber-reinforced plastic material impregnated by a technique involving, for example, bringing a low-molecular-weight polyamide resin in a powder state having reduced numbers of terminal amino groups and carboxyl groups into contact with a reinforcing base material. However, because of the low molecular weight of the polyamide resin used, mechanical physical properties of the FRP are somewhat low. In addition, its molding temperature is as high as 290° C., and hence temperature increase and temperature decrease require time. Accordingly, the technique is unsuitable for producing an FRP molded article with good productivity.
In addition, in Patent Literature 3, there is a disclosure of a novel phenoxy resin having high moldability and high heat resistance, and there is also a disclosure that a reinforcing fiber base material can be impregnated with the phenoxy resin by a hot-melt method or a solvent method to produce an FRP molding prepreg. However, this technique essentially requires a special fused ring structure-containing phenoxy resin. The phenoxy resin has a glass transition temperature (Tg) of at most about 150° C., and hence is insufficient for application to a member to be used under a harsh environment in an automobile or the like. Also when applied to a vehicle body, the phenoxy resin cannot withstand the temperature during baking finishing. Therefore, primer treatment or the like is separately required, and hence an extra step required in a production process. In addition, the hot-melt method is generally difficult when the resin for impregnation has a large molecular weight. The phenoxy resin has a molecular weight of from about 30,000 to about 70,000, and hence application of the hot-melt method thereto is considered to be difficult. In addition, in the solvent method, a large amount of a solvent is used in order to improve an impregnating property, and hence the prepreg to be obtained has a drawback in that its tackiness is strong owing to a residual solvent, resulting in difficulty in workability. Further, there is no disclosure of Example or Comparative Example of the prepreg using the phenoxy resin.
As described above, the FRP molding material is required to be capable of melting at relatively low temperature to significantly shorten a molding time (high moldability and high productivity), and at the same time, is required to be capable of being shaped into a complicated shape like a housing of a notebook PC or a liquid crystal tablet, and to provide an FRP molded article having excellent characteristics (high toughness, high heat resistance, and long lifetime).
In view of the foregoing, a technique involving increasing the Tg of a low-Tg thermoplastic resin by a cross-linking reaction utilizing heat at the time of molding processing is currently under consideration. For example, in Patent Literature 4, there is a disclosure of a phenoxy resin composition whose heat resistance can be improved by adding a cross-linking agent to a phenoxy resin, which is a thermoplastic resin, and applying heat thereto to cause a cross-linking reaction, and it is mentioned that the phenoxy resin composition can be used in molding processing, though an optical part is produced. However, even in this material, thermal history at the time of the molding processing is not enough for the cross-linking reaction for improving the Tg, and hence heat treatment for from 30 min to 60 min is separately required. In addition, in kneading of the material in a stage prior to the molding, a reaction with the cross-linking agent present therein is liable to proceed to cause gelation, and hence there is a problem of how to impregnate, with the material, a reinforcing fiber base material.